Ray string calculation - javascript for php

Question

This example and java script code refer to link text
Take a look at the section on edge lines.

---

Given the starting point and the distance d along the constant bearing θ, this will calculate the destination. If you maintain a constant bearing along the line of the stream, you will gradually scroll to one of the poles.

Formula

α = d/R (angular distance) lat2 = lat1 + α.cos(θ) Δφ = ln(tan(lat2/2+π/4)/tan(lat1/2+π/4)) [= the 'stretched' latitude difference] if E:W line q = cos(lat1) otherwise q = Δlat/Δφ Δlon = α.sin(θ)/q lon2 = (lon1+Δlon+π) % 2.π − π where ln is natural log and % is modulo, Δlon is taking shortest route (<180°), and R is the earth's radius 

JavaScript :

 lat2 = lat1 + d*Math.cos(brng); var dPhi = Math.log(Math.tan(lat2/2+Math.PI/4)/Math.tan(lat1/2+Math.PI/4)); var q = (!isNaN(dLat/dPhi)) ? dLat/dPhi : Math.cos(lat1); // EW line gives dPhi=0 var dLon = d*Math.sin(brng)/q; // check for some daft bugger going past the pole, normalise latitude if so if (Math.abs(lat2) > Math.PI/2) lat2 = lat2>0 ? Math.PI-lat2 : -(Math.PI-lat2); lon2 = (lon1+dLon+Math.PI)%(2*Math.PI) - Math.PI; 

---

I am trying to convert it to php syntax, but I am not getting the desired result. My latitudinal part is working fine. I also included my test data.

---

MY PHP CODE

 // test data $R = 6371; $tlatitude = 50.7; $tlongitude = -105.214; $theading = 124; $d = 50; $projlat = $tlatitude + rad2deg(($d/$R)*COS(deg2rad($theading))); //Δφ = ln(tan(lat2/2+π/4)/tan(lat1/2+π/4)) $delta_phi = log(tan(deg2rad($projlat/2) + pi()/4)/(tan(deg2rad($tlatitude/2) + pi()/4))); //q = Δlat/Δφ $delta_lat = deg2rad($projlat - $tlatitude); $q = $delta_lat/$delta_phi; //Δlon = α.sin(θ)/q $delta_long = rad2deg($d/$R*sin(deg2rad($theading))/$q); $projlong = $tlongitude + $delta_long; 

I get $projlong = -104.84

according to the link page, the answer should be -104.63 .

Now I am trying to get this to work, ignoring the east-west and the pole possibilities.

Answer 1

I had some problems calculating distances, where my errors would increase somewhat after a while. I found that if I cast (double) in my code, the accuracy will increase. I did not look at C code in PHP to find out what caused this. I could then abandon my BC version of the code.

If you need extra precision, check out the BC functions in PHP. http://php.net/manual/en/book.bc.php

Also, remember that the order in which the computer performs the calculations will affect your accuracy. That is, the calculation is lower

 $d/$R*sin(deg2rad($theading))/$q 

will not give the same result as

 $d*sin(deg2rad($theading))/$q/$R 

and it can also give a third result

  $d*sin(deg2rad($theading))/($q*$R) 

This is due to limited accuracy for numbers close to zero (0) on computers.

Answer 2

javascript has better accuracy than php look at this joke http://ru2.php.net/manual/en/function.doubleval.php

I needed to check IBAN with the Moon algorithm. My javascript code worked beautifully. But my php failed, so after some research, I found a joke and had to recode the basic operations (add, subo, calculate, divide, modulo) based on a string, not a number.

You may also need to transcode it to get the expected accuracy. We should not use php for high precision computing.